A brushless doubly-fed induction motor (BDFIM) has two sets of stator windings for two p-poles and two q-poles. The rotor winding has a nested cage with 2(p+q) poles. It produces a motor control with a relatively narrow range of speed control. The existing extended rotor cage technology has a drawback in that the both rotational and slip energy are transferred in a cluttered approach to energy transfer.
An induction machine may be viewed as a transformer with its stator as the primary and the rotor as the secondary. A slip-ring wound-rotor induction motor, with a secondary winding is connected through a set of slip rings and brushes, has been known for decades. By changing the resistance connected to the brushes, the starting current and the speed of the motor can be changed. However, maintenance of a motor with slip rings and brushes is expensive.
It is generally agreed that the most significant energy savings for electric motor drives comes from the adjustable speed drive and that the motor plays a relatively less significant role. The high cost of adjustable speed drives fed by an adjustable-frequency inverter discourages many potential users. There are many other known adjustable speed methods. The brushless doubly-fed motor (BDFM) provides an adjustable-speed control having a lower initial cost than other alternatives.
Hsu, U.S. Pat. No. 6,310,417, issued Oct. 30, 2001, disclosed a hybrid-secondary uncluttered induction machine that has a significant potential to lower the cost of adjustable-speed drives. In addition to speed control below synchronous speed, this machine may also be operated above synchronous speed.
The term “hybrid secondary” as it relates to such a machines implies that several secondary circuits can be used in various combinations for different applications. Examples of such secondary circuits are a variable resistance circuit, an inverter circuit for doubly-fed operation, and a generator circuit.
The term “uncluttered coupling” relates to a stator and rotor that couple slip energy. In an induction motor, the speed of the rotating stator field equals the sum of 1) the speed of the rotating rotor field plus 2) the mechanical rotation speed of the rotor. With the motor running at maximum torque and close to synchronous speed, rotor speed is high and slip (the difference between the speed of the rotating stator field and the rotational speed of the rotor) is small, about 3 to 10 percent, and the slip frequency induced in the rotor is small, perhaps two to six cycles per second for a 60 Hz motor.
To couple only slip energy, the stator and rotor have coils that run circumferentially, sometimes referred to as “peripherally,” around the axis of rotor rotation. The peripheral coils of the rotor and stator are magnetically coupled. The rotor coil rotates and carries a slip-frequency current. Because the rotation does not change the total magnetic flux linking both the rotor and stator coils, no electromotive force (emf) is induced in the stator coil due to the rotation of the rotor coil. This “uncluttered coupling” allows only the slip energy power corresponding to the slip-frequency currents to be transferred between the rotor and stator coils of the transformer.
It is desired to make such a machine that is more compact and has fewer parts while still providing a source of slip energy for speed control.